The invention relates to a method and also an apparatus for applying an integrated circuit to a carrier element.
In order to produce a mechanical and electrical incorporation of an integrated circuit within the circuit of an electronic device, it is possible to connect integrated circuits to a carrier element in a generally known way, in a manner that saves space and material, by use of xe2x80x9cchip-size/scale-packaging (CPS)xe2x80x9d or the like. In this case, by way of example, a flexible rewiring sheet can be used as the carrier element. The rewiring sheet may contain a polyimide film having a thickness of approximately 50 micrometers with copper conductor tracks having a thickness of approximately 15 micrometers. In order to minimize thermomechanical stresses on account of operation-dictated or mounting-dictated temperature differences between the integrated circuit and the carrier element, an interspace between the two elements is usually filled with a compensating layer. The compensating layer takes up the undesirable stresses, thereby effectively reducing in particular the risk of tearing of electrical lines connecting the integrated circuit to the carrier element at the contact-mating points.
International Patent Disclosure WO 97/40958 discloses a method for applying an integrated circuit to a carrier element which underfills the interspace to create the compensating layer that takes up stress. For this purpose, the integrated circuit is applied by use of spacer silicone spots on the carrier element in order to define the width of the interspace. Afterward, the electrical connection between the integrated circuit and the carrier element is produced by breaking out conductor track sections from prefabricated perforations in the carrier element, followed by fastening to corresponding contact points on the integrated circuit. In this case, the fastening is effected by a xe2x80x9cbonding methodxe2x80x9d. Next, with pressure shut off, an initially flowable material is introduced into the interspace by the action of negative pressure, which material finally cures to form the compensating layer.
In this case, it is disadvantageous that complex preconditions in terms of equipment are necessary for underfilling the integrated circuit by the action of negative pressure in order to guarantee that the interspace is completely filled. Furthermore, the flowability of the material of the compensating layer must always be ensured during the underfilling process. Therefore, this known solution for applying the integrated circuit to the carrier element requires a multiplicity of method steps that altogether cause a time-consuming mounting process.
International Patent Disclosure WO 97/27624 discloses an apparatus for underfilling the interspace which requires fewer production steps in comparison with the above. A mold is placed like a cup over the integrated circuit that is connected to the carrier element electrically and mechanically at a distance, the mold surrounds the integrated circuit and the edge of the mold bears on the carrier element. An injection device disposed thereon promptly forces a flowable material into the interspace, the flowable material forming the compensating layer in the cured state. Since the underfilling is effected by excess pressure in this respect, it is not necessary here to produce a negative pressure in the interspace. Although comparatively fewer production steps are necessary using the apparatus, so that a somewhat shorter mounting time can be realized, the apparatus for underfilling is nonetheless altogether extremely costly and complicated in its construction.
It is accordingly an object of the invention to provide a method and an apparatus for applying a semiconductor chip to a carrier element that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which enables a simplification in the production while achieving an optimal stress-absorbing function of the compensating layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for applying a first device to a second device. The method includes providing an integrated circuit having a lower contact area and a carrier element having conductor tracks. A curable compensating layer having an initially pasty consistency is coated with full coverage onto the lower contact area of the integrated circuit or onto the carrier element. The integrated circuit is subsequently joined in an adhering manner, due to the curable compensating layer, to the carrier element after relative alignment. An electrical connection is produced between the integrated circuit and the conductor tracks of the carrier element via electrical lines surmounting a thickness of the curable compensating layer. The curable compensating layer is cured which results in an increased volume of the curable compensating layer.
The invention thus advantageously departs from the underfilling technique employed in the prior art and takes a new path in that the coating of the material which forms the compensating layer in the cured state is effected before the electrical contact-making. As a result, in particular the first method step of the interspace-forming attachment of the integrated circuitxe2x80x94for example by silicone spotsxe2x80x94is entirely obviated, thereby considerably simplifying production. The basis for forming the compensating layer is also advantageously produced at the same time as the application of the integrated circuit to the carrier element. Reliable filling of the interspace is guaranteed by the curing of the compensating layer with an increase in volume. Overall, the method according to the invention shortens the mounting time.
A further measure that improves the invention consists in the electrical lines initially being fastened in an untautened manner between the integrated circuit and the carrier element. The electrical lines are tautened, i.e. straightened up in disposition relative to the surface of the carrier element, only during the increase in volume of the compensating layer. In this case, the length of the tautened lines corresponds to a minimum thickness of the compensating layer that must be present in order to be able to compensate for the stresses that occur. In the untautened state, the electrical lines are configured for example as arcuately shaped, loose small copper strips.
The increase in volume of the compensating layer can be brought about in various ways. This can be effected by introducing a gas during the coating process or during the curing process. Alternatively, the volume of the compensating layer can also be increased by a substance which liberates gas (during the curing process) and is admixed with the base material.
Preferably, the volume-increasing process can be initiated by increasing the temperature of the compensating layer during the curing process or irradiating the compensating layer with ultraviolet light or in another suitable way. A heating device or a UV lamp can be used for this purpose. In addition, a temperature increase can also be effected by an exothermic reaction of the material of the compensating layer during the curing process. Separate temperature-increasing devices are not necessary in this case.
It is advantageous if, in the method according to the invention, the electrical lines are fastened by a conventional bonding method, in order to ensure rapid and reliable contact-making of the integrated circuit with the geometrical dimensions given here. Thus, by way of example, xe2x80x9clead bondingxe2x80x9d, xe2x80x9claser bondingxe2x80x9d or xe2x80x9cthermocompression bondingxe2x80x9d can be employed as the bonding method.
The above-described method for applying an integrated circuitxe2x80x94which can also be fabricated by chip wafer technologyxe2x80x94to a carrier element can preferably be carried out using an apparatus which contains a device for substantially full-coverage coating of a curable, adhesive compensating layer onto a lower contact area of the integrated circuit. In this case, the coating device may be a type of pressure-actuable cylinder configuration with an outlet opening or a configuration for printing. Furthermore, the apparatus contains a placement devicexe2x80x94for example a pick-and-place devicexe2x80x94for joining together, in an aligned manner, the integrated circuit with the carrier element by the adhering compensating layer. A wiring devicexe2x80x94in particular a bonding devicexe2x80x94serves for electrically connecting the integrated circuit to conductor tracks of the carrier element via electrical lines surmounting the thickness of the compensating layer. The wiring device forms the electrical lines, which are preferably configured as small copper strips, from the conductor tracks of the carrier element. As an alternative to this, the method of xe2x80x9cwire bondingxe2x80x9d or the like can also be employed in order to produce the electrical connection between the integrated circuit and the carrier element. If appropriate, a separate devicexe2x80x94for example a heating devicexe2x80x94may be necessary for curing the compensating layer with an increase in volume if this is not contained in the chemical reaction of the material itself which forms the compensating layer.
A foamable silicone material or a foamable epoxide material is preferably used as material for the compensating layer and is injected by the device for coating. In addition, other materials having the desired foamable properties for forming the stress-absorbing compensating layer are also conceivable.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for applying a semiconductor chip to a carrier element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.